The question whether assist-control ventilation (A/C) results in more frequent or more severe respiratory alkalosis than intermittent mandatory ventilation (IMV) is often raised. We prospectively compared the respiratory rates and arterial blood tensions of 18 critically ill patients with respiratory failure of diverse causes who were mechanically ventilated for 1 h with each of these ventilatory modes. Each patient served as his own control. We found that after 1 h of IMV the average pH was 7.42 +/- 0.2 (mean +/- SEM), after 1 h of A/C the pH was 7.45 +/- 0.01 (p less than 0.005), the average PaCO2 during IMV was 40.7 +/- 1.8, the average PaCO2 during A/C was 37.9 +/- 1.6 (p less than 0.001), the average respiratory rate during IMV was 21 +/- 2.0, and the average respiratory rate during A/C was 15 +/- 2.0 (p less than 0.001). One patient became alkalemic (pH 7.55) during A/C. These pH and PaCO2 changes were not associated with any adverse clinical sequelae. We conclude that the responsible physician should be guided by factors other than control of pH in choosing the mode of mechanical ventilation for most patients.